This invention relates to an improvement on a discharge ring supporting structure of an adjustable-blade axial-flow turbine.
A conventional bulb-type axial-flow turbine, which is a kind of an adjustable-blade axial-flow turbine, comprises a bulb axially disposed in a water flow and containing therein a generator and a main shaft, a runner fitted to the end of the bulb and connected to the main shaft, an outer casing surrounding the bulb for guiding water flow into the runner, adjustable wicket gates controlling the water flow into the runner, an outer wicket gate case, and a discharge ring surrounding the runner. The runner has a plurality of adjustable blades, the tips of which are formed such that the rotational trace of the tips of the blades is partially spherical so that the tips are disposed on the same partially spherical rotating trace even when the blades are opened and closed. The discharge ring is disposed axially between the outer wicket gate case and a draft tube in order to minimize water flow loss, the discharge ring has a partially spherical inner surface facing the tips of the runner blades with a small seal gap therebetween that, being coaxial of the partial spherical shape, forms a throat portion immediately downstream of the runner blades. The throat portion has a smaller diameter than that of the runner blades.
The discharge ring is secured to the outer wicket gate case on the upstream side, and on the downstream side a gap is provided between the discharge ring and the draft tube to make it easy to assemble and disassemble the discharge ring. The gap is sealed by a loose flange mechanism to prevent water leakage.
The combined outer wicket gate case and the discharge ring is exposed without being covered with a material such as concrete. The outer wicket gate case and the discharge ring are deflected by their own weight and water weight, with the upstream side end of each of the outer wicket gate case and the discharge ring acting as a fulcrum. The deflection becomes larger as the capacity of the turbine increases. Originally, the axial-flow turbine was used for low head. The shell thickness of tubular structures employed in the axial-flow turbine is thinner than the other kinds of water turbines, so that the rigidity of the tubular structures is small. Consequently its spring constant and its natural vibration frequency also are small. When the natural vibration frequency accords with the vibration frequency f.sub.r caused by water pressure, resonance takes place. This sometimes causes large troubles such as structure rupture or contact between the runner blades and the discharge ring to occur. Further, when the turbine runs under partial load, or in overload, swirling flow is easily caused downstream of the runner, whereby eddies and water pressure vibration take place. Therefore, vibration is easily caused in the discharge ring. In particular, since the adjustable-blade axial-flow turbine has a discharge ring exposed without being covered with concrete as above-mentioned, and since the turbine is disposed axially, the tubular structure is likely to be bent. Its rigidity against deflection and spring constant can not be made sufficiently high, so that resonance and unusual vibration take place sometimes.
The conventional discharge ring is supported by a pair of supporting feet and concrete columns connected to the supporting feet and a concrete floor or base. The supporting feet each are provided on the periphery of the discharge ring at the throat portion.
The spring constant K.sub.c, in the vertical direction, of the supporting feet and the concrete columns, which are integrated with the discharge ring, is usually made higher than the bending spring constant K.sub.b of the axial tubular structure constructed of the discharge ring and the outer wicket gate case. The total spring constant K.sub.t is given by the following equation; EQU K.sub.t =(K.sub.c .multidot.K.sub.b)/(K.sub.b +K.sub.c).
K.sub.c is much larger than K.sub.b, so that K.sub.t is nearly equal to K.sub.b. Namely, the spring constant for the bending of the tubular structure relies only on the bending spring constant. The supporting feet and concrete columns each are not adjustable in spring constant after their manufacturing, so that if they are made such that the natural vibration frequency of the tubular structure determined by the total spring constant K.sub.t is equal to the water pressure vibration frequency fr, the structure is brought into resonance. This can cause the above-mentioned troubles.
At present, it is not sufficient to precisely anticipate in the design stage whether or not the resonance takes place. Therefore, there is the probability that such troubles can occur relatively often. As mentioned above the supporting structure comprising the supporting feet of the discharge ring and their concrete columns can not be changed in the rigidity, so that there is no way for shifting the resonance point of the tubular structure comprising the discharge ring and the outer wicket gate case other than reinforcing the structure. This reinforcement operation consumes much time and money, further it accompanies technical difficulty such that deformation caused by the reinforcement must be made small.
Further, in a transitional operation of the water turbine such as starting or emergent stopping, a large magnitude of the vibration takes place temporarily.
In the conventional structure, the vibrator, that is, the supporting structure and the tubular structure has no factor for raising a damping effect. The damping effect relies only on solid body damping due to internal friction caused in the material of the structure, so that the vibration of a large magnitude which temporarily takes place can not be easily damped and the vibrator is maintained under a dangerous condition for a long time. In this period, it is likely that the runner blades will contact with the discharge ring.
Further, the conventional structure has an economical drawback in addition to the problem relating to the vibration.
As mentioned above, the seal gap is formed between the blade tips and the discharge ring facing the tip. The seal gap has a partially spherical shape which has the throat portion on the immediately downstream side of the blades. The radius of the discharge ring at the throat portion is smaller by G.sub.1 than each of the blades at the throat portion. In the disassembly of the turbine, for example, the runner is removed from the bulb by horizontally moving the runner. The runner, however, contacts with the discharge ring at the throat portion so that the runner can not be removed from the bulb. Therefore, the conventional discharge ring is divided by a vertical plane into two parts, a left half and a right half. In advance of the removal of the runner, the left and right halves of the discharge ring are moved horizontally enough not to contact with the runner. Therefore, a housing in a power plant needs the width W given by the following equation: EQU W=2.times.(E+B+h)+D
wherein
D: the maximum diameter of the runner blades, PA1 E: the distance between the housing and the one half of the discharge ring when the left and right halves have been moved horizontally enough to remove the runner, PA1 B: the scale of the left or right half in the horizontal and radial direction, and PA1 h: the distance between the runner blades and the discharge ring when the runner is removed.
For the conventional turbine, it is necessary to make the housing width large for assembling and disassembling, so that the turbine is remarkably uneconomical.
Further, the mounting floor for the discharge ring is the lowest in the housing of the power plant. On the floor, various devices such as waste water pumps, feed water pumps, leakage oil tanks, lubrication oil tanks, etc. which are necessary to operate the turbine. In order to avoid contact with these various devices, the devices are shifted in the positions where they are not in contact with the discharge ring when horizontally moved, so that the housing is made larger by such a scale. Additionally, the discharge ring removed from the bulb is stored in an assembling room of the power plant, so that a wide space is necessary for storage of the discharge ring in addition to the other parts during the overhaul.